I dont use the Spyder but I'm surprised it has no facility to give you feedback on monitor brightness.

Basically, the brightness has to be set so that your prints, under desired viewing conditions, match your screen. Taht will probably mean a brightness of 90-110 cd/m2 (YMMV). Most monitors are WAY TOO bright and usually, a correctly set monitor (for photo retouching) seems kinda 'dim' relative to what you normally use/see.